This 2016 Singapore review was on the role of telomerase in cancers. From its background section:

“Telomeres are conserved, repetitive sequences located at the ends of eukaryotic chromosomes which protect the integrity of genomic DNA. DNA polymerase is unable to replicate the 5′ [carbon number] ends of chromosomes, hence, cells require a RNA dependent DNA polymerase called telomerase to synthesize DNA on the lagging strand. Telomerase activity is tightly regulated and seen mainly in germ cells, stem cells and some immune cell types which have high proliferative needs.

In contrast, somatic cells do not display detectable telomerase activity. As a result, the chromosomes of normal somatic cells shorten 50–200 bp [base pair] each replication at the telomeres due to the problem of end replication. Thus, somatic cells are eventually burdened with DNA damage, replication crisis, cellular senescence or apoptosis and can divide only limited number of times, whereas cells that have active telomerase possess unlimited proliferative potential.”

The main section of the review described the details of how:

“Reactivation of telomerase has been considered as a strategy for telomere maintenance and is a major hallmark of cancer. Although the major function of telomerase is thought to be telomere elongation, accumulating evidence has suggested that it can modulate expression of various genes which affect cancer progression and tumorigenesis.”

http://link.springer.com/article/10.1007/s00018-016-2146-9/fulltext.html “Reactivation of telomerase in cancer”